This invention relates to a measuring apparatus for determining the position of a movable element with respect to at least two spaced reference means, which movable element can emit a propagation energy wave to be received by the reference means, the propagation energy wave received being passed from each of the reference means to a processing unit for determining the position of the movable element with respect to the reference means.
Such a measuring apparatus for determining the position of the movable element with respect to the reference means is known per se. For this purpose, the movable element emits a sound wave which is received by a pair of spaced reference means in the form of Micro phones. From the difference in propagation time of the sound wave from the movable element to each of the microphones the position of the movable element is determined by means of a processing unit.
A drawback of such a measuring apparatus operating on sound waves is that the sound waves require a medium, such as air, to be able to propagate. Thus, for instance, air displacement, e.g. draught, will affect the direction of sound displacement as well as the sound volume. Furthermore, sound waves are susceptible to temperature differences, as the propagation speed is proportional to the ambient temperature. Therefore, draught and ambient temperature adversely affect the correctness of the measurement.
A further drawback is that the microphones will detect not only the sound waves from the movable element but all other sound waves, e.g. sounds of human voices, machines etc.
As the measurements are generally conducted under conditions at which the above drawbacks are often more or less present, such a measuring apparatus is not always equally accurate, and the inaccuracy will certainly increase as the distance from the movable element to the microphones increases.
Another drawback of this known measuring apparatus is that the sound waves are generally obtained by induction waves, which implies a relatively high induced electromotive force. As such an induced electromotive force may negatively affect the operation of adjacent electronic devices, the induction-producing circuit is supplied as a separate unit, which is laborious. Moreover, under certain conditions a measurement with sound waves is not useful, e.g. when the movable element is scanning an object in the form of a printed circuit board. The fact is that induction waves may adversely affect the electronic devices of the printed circuit board.